Ultrafast decoherence in solid-state high-harmonic generation induced by nuclear-electronic entanglement.

High-harmonic generation (HHG) has provided groundbreaking insights into nonequilibrium dynamics involving strong light-matter interactions. However, intrinsic quantum effects of condensed matter, such as electronic coherence and its interplay with nuclear degrees of freedom─one of the most fascinating aspects of quantum mechanics─have largely been overlooked. Here, we explore the nuclear quantum effects (NQEs) on electronic coherence in solid-state HHG. Unlike classical nuclei, the strong delocalization of the nuclei caused by zero-point vibrations induces electronic decoherence on an ultrafast (attosecond to femtosecond) timescale through nuclear-electronic entanglement. In this manner, NQEs not only contribute to the ultrafast dephasing of HHG but also suppress the interband electron trajectory and thus switch the dominant mechanism from interband to intraband HHG in solids. This yields measurable signatures in HHG spectroscopy, enabling direct probe of coherence time and nuclear wave packets information in materials.